Cross reference is made to U.S. patent application Ser. No. 09/227,434, filed Jan. 8, 1999, which is assigned to the assignee of the present invention.
This invention relates to electricity meters such as used by commercial, industrial, or residential customers of power utility companies and, more particularly, to a revenue accuracy meter having various operational capabilities such as power quality measurement and/or energy management.
Utility power distribution generally starts with generation of the power by a power generation facility, i.e., power generator or power plant. The power generator supplies power through step-up subtransmission transformers to transmission lines. To reduce power transportation losses, the step-up transformers increase the voltage and reduce the current. The actual transmission line voltage conventionally depends on the distance between the subtransmission transformers and the users or customers. Distribution substation transformers reduce the voltage from transmission line level generally to a range of about 2-35 kilo-volts (xe2x80x9ckVxe2x80x9d). The primary power distribution system delivers power to distribution transformers that reduce the voltage still further, i.e., about 120 V to 600 V.
For background purposes, and future reference herein, an example of a power utility distribution system as described above and understood by those skilled in the art is illustrated in FIGS. 1A and 1B of the drawings. Power utility companies, and suppliers thereto, have developed systems to analyze and manage power generated and power to be delivered to the transmission lines in the primary power distribution system, e.g., primarily through supervisory control and data acquisition (xe2x80x9cSCADAxe2x80x9d). These primary power distribution analyzing systems, however, are complex, expensive, and fail to adequately analyze power that is delivered to the industrial, commercial, or residential customer sites through the secondary power distribution system.
Also, various systems and methods of metering power which are known to those skilled in the art are used by commercial, industrial, and residential customers of power utility companies. These power metering systems, however, generally only measure the amount of power used by the customer and record the usage for reading at a later time by the utility power company supplying the power to the customer. A revenue accuracy meter is an example of such a metering system conventionally positioned at a customer site to receive and measure the amount of power consumed by the customer during predetermined time periods during a day.
Conventionally, electric power is delivered to industrial, commercial, and residential customers by local or regional utility companies through the secondary power distribution system to revenue accuracy type electricity meters as an alternating current (xe2x80x9cACxe2x80x9d) voltage that approximates a sine wave over a time period and normally flows through customer premises as an AC current that also approximates a sine wave over a time period. The term xe2x80x9calternating waveformxe2x80x9d generally describes any symmetrical waveform, including square, sawtooth, triangular, and sinusoidal waves, whose polarity varies regularly with time. The term xe2x80x9cACxe2x80x9d (i.e., alternating current), however, almost always means that the current is produced from the application of a sinusoidal voltage, i.e., AC voltage.
In an AC power distribution system, the expected frequency of voltage or current, e.g., 50 Hertz (xe2x80x9cHzxe2x80x9d), 60 Hz, or 400 Hz, is conventionally referred to as the xe2x80x9cfundamentalxe2x80x9d frequency, regardless of the actual spectral amplitude peak. Integer multiples of this fundamental frequency are usually referred to as harmonic frequencies, and spectral amplitude peaks at frequencies below the fundamental are often referred to as xe2x80x9csub-harmonics,xe2x80x9d regardless of their ratio relationship to the fundamental.
Various distribution system and environmental factors, however, can distort the voltage waveform of the fundamental frequency, i.e., harmonic distortion, and can further cause spikes, surges, or sags, and other disturbances such as transients, time voltage variations, voltage imbalances, voltage fluctuations and power frequency variations. Such events are often referred to in the art and will be referred to herein as power quality disturbances, or simply disturbances. Power quality disturbances can greatly affect the quality of power received by the power customer at its facility or residence.
These revenue accuracy metering systems have been developed to provide improved techniques for accurately measuring the amount of power used by the customer so that the customer is charged an appropriate amount and so that the utility company receives appropriate compensation for the power delivered and used by the customer. Examples of such metering systems may be seen in U.S. Pat. No. 5,300,924 by McEachern et al. titled xe2x80x9cHarmonic Measuring Instrument For AC Power Systems With A Time-Based Threshold Meansxe2x80x9d and U.S. Pat. No. 5,307,009 by McEachern et al. titled xe2x80x9cHarmonic-Adjusted Watt-Hour Meter.xe2x80x9d
These conventional revenue accuracy type metering systems, however, have failed to provide information about the quality of the power received by a power customer at a particular customer site. Power quality information may include the frequency and duration of power quality disturbances in the power delivered to the customer site. As utility companies become more and more deregulated, these companies will likely be competing more aggressively for power customers, particularly heavy power users, and therefore information regarding the quality of the power received by the power customer is likely to be important.
For example, one competitive advantage that some utility companies may have over their competitors could be that their customers experience relatively few power quality disturbances. Similarly, one company may promote the fact that it has fewer times during a month that power surges reach the customer causing potential computer systems outages at the customer site. Another company may promote that it has fewer times during a month when the voltage level delivered to the customer is not within predetermined ranges which may be detrimental to electromagnetic devices such as motors or relays. Previous systems for measuring quality of power in general, however, are expensive, are bulky, require special set up and are not integrated into or with a revenue accuracy meter. Without a revenue accuracy metering system that measures the quality of the power supplied to and received by the customer, however, comparisons of the quality of power provided by different suppliers cannot readily be made.
One solution to the above described problems is proposed by U.S. Pat. No. 5,627,759 to Bearden et al. (hereinafter the xe2x80x9cBearden patentxe2x80x9d), which is assigned to the assignee of the present invention and incorporated herein by reference. The Bearden patent describes a revenue accurate meter that is also operable to, among other things, detect power quality events, such as a power surge or sag, and then report the detection of the power quality event to a utility or supplier.
One of the useful features of the meter disclosed in the Bearden patent is waveform capture. The meter of the Bearden patent is operable to obtain waveform information regarding the voltage and/or current waveform at about the time a power quality event is detected. Such a feature is advantageous because the captured waveform may be analyzed to help determine potential causes of the event, the severity of the event, or other pertinent data.
One use of meters that have the waveform capture capabilities of the Bearden patent is to analyze the waveforms of several such meters after a power quality event to determine the performance of a certain portion of the network at the time of the power quality event. For example, if a power swell occurs over a portion of the power distribution system, then the utility may obtain captured waveforms from various meters on that portion of the network. The utility may then obtain information on how the power swell propagated through the network, as well as other information, by comparing the waveforms from the various meters.
One difficulty of performing analysis on the captured waveforms of several meters is temporally aligning the captured waveforms. In particular, to benefit from comparing the waveforms from several meters after a power quality event, it is important to temporally align or synchronize the captured waveforms. However, commonly used electronic clocks in electronic meters are not highly synchronized to each other, or in fact, to any external equipment. In the past, clock circuits within revenue meters have been calibrated periodically using the line voltage signal, which oscillates at 60 Hz. While such a practice increases the accuracy of the meter clock circuits, it cannot provide several dispersed meters with sufficiently accurate synchronization to produce reliable comparative waveform analysis.
What is needed, therefore, is a power measurement device which is coupled with a revenue accurate meter having a mechanism for synchronizing its internal clock with the clocks of one or more like meters and/or utility equipment.
The present invention addresses the above needs, as well as others, by providing a source of externally-generated precision time standard information within a revenue accurate meter. The time standard information, may, for example, be obtained from a global position satellite (GPS) generated time standard, a WWV time standard, or an IRIG-type time standard. A meter having such a source of externally-generated precision time standard information could then have its internal clock highly synchronized with other equipment, regardless of the location of such equipment.
An exemplary apparatus according to the present invention includes an electrical energy meter having a voltage digitizing circuit, a current digitizing circuit, a metering circuit, a clock circuit, and a source of externally-generated time standard information. The voltage digitizing circuit is operable to obtain analog line voltage information and generated digital line voltage information therefrom. Similarly, the current digitizing circuit is operable to obtain analog line current information and generate digital line current information therefrom. The metering circuit is operable to receive the digital line voltage information and the digital line current information and generate metering information therefrom. The clock circuit is operable to generate calendar/clock information, said clock circuit having a calibration input for receiving precision time calibration information. The source of externally-generated time standard information is operably coupled to provide precision time calibration information to the calibration input of the clock circuit.
The exemplary apparatus described above may further optionally include waveform capture functionality. Such a device could then capture waveforms and associate clock/calendar information with the captured waveform, wherein the clock/calendar information is calibrated to the externally generated time standard. If several such devices are incorporated into a power distribution network, the waveforms captured by several such devices would be highly synchronized to each other, thereby allowing analysis of the distribution network performance in the event of a power quality event.
The above features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings.